Proton Conductivity in Nafion

Proton conductivity in Nafion-117 increases with increase in water content and temperature. The experimental fit of the proton conductivity data (Q -cm) as a function of temperature and membrane water hydration is given as (Springer et al., 1991) 

Variation of proton conductivity of Nafioii-117 with temperature for different water content. 

Variation of proton conductivity of Nafion with water content at 303 K. 

Estimate the proton conductivity for Nafion membrane with 15% moisture in the air stream with an operating pressure and temperature of 1 atm and 70°C, respectively. 

At an operating temperature of 70°C, the corresponding water saturation pressure is = 31.19 kPa. Based on this water vapor pressure, the water activity ratio with 15% moisture content is given as 

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Cahan BD, Wainright JS (1993) AC impedance investigations of proton conduction in Nafion . J Electrochem Soc 140 L185-6... 

A precondition for proton conductivity in Nafion is uptake of sufficient water into the membrane and prevention of its drying out during operation. The protons diffuse through water channels in the membrane to the cathode or are transported... 

Proton conduction in Nafion requires water to ionize the sulfonic acid groups and establish percolation paths through the membrane [2]. The water uptake by the PEM, X = number of water molecules per sulfonic acid residue, and the membrane resistivity, R, are functions of the water activity, aw they have negligible dependence on temperature [21, 22]. 

FIGURE 5.15 In situ method for measuring proton conductivity in Nafion membrane by using EC-AFM. The applied voltage induces water oxidation at the electrode. TTie protons are only transported through the membrane when the conductive AFM tip makes contact with an ion channel. These protons are used for the ORR at the AFM tip. [11,66,67]. (For color version of this figure, the reader is referred to the online version of this hook.)... 

Electrolytes for Electrochromic Devices Liquids are generally used as electrolytes in electrochemical research, but they are not well suited for practical devices (such as electrochromic displays, fuel cells, etc.) because of problems with evaporation and leakage. For this reason, solid electrolytes with single-ion conductivity are commonly used (e.g., Nafion membranes with proton conductivity. In contrast to fuel cells in electrochromic devices, current densities are much lower, so for the latter application, a high conductivity value is not a necessary requirement for the electrolyte. 

GDE s may be interesting for synthesis cells as depolarized electrodes (e.g. [48]). A hydrogen-consuming anode will work at a low potential that avoids undesired anodic oxidations (e.g. no chlorine evolution in presence of chlorides). In order to reject an excess of the electrolyte from the GDE structure, a proton-conducting membrane (Nafion ) between the GDE and the electrolyte can be used ( Hydrina , De Nora Spa. [49]). 

Figure 18 shows the temperature dependence of the proton conductivity of Nafion and one variety of a sulfonated poly(arylene ether ketone) (unpublished data from the laboratory of one of the authors). The transport properties of the two materials are typical for these classes of membrane materials, based on perfluorinated and hydrocarbon polymers. This is clear from a compilation of Do, Ch 20, and q data for a variety of membrane materials, including Dow membranes of different equivalent weights, Nafion/Si02 composites ° ° (including unpublished data from the laboratory of one of the authors), cross-linked poly ary lenes, and sulfonated poly-(phenoxyphosphazenes) (Figure 19). The data points all center around the curves for Nafion and S—PEK, indicating essentially universal transport behavior for the two classes of membrane materials (only for S—POP are the transport coefficients somewhat lower, suggesting a more reduced percolation in this particular material). This correlation is also true for the electro-osmotic drag coefficients 7 20 and Amcoh...

The reason why atomistic modeling study of PEFCs is considered as being in its initial stage of progress is that the system size and timescale in the simulations are often very limited. With the AIMD approach that is considered to be exact can reveal some very important mechanisms for OER, oxidation of CO and methanol, and proton transfer in Nafion. However, because the simulations are conducted with very small system (<200 atoms) and with a timescale of several picoseconds, it may be an overstatement when one claims that a phenomenon observed experimentally can be explained with mechanisms found in the simulations. 

Verbragge and Hill [77] have compared protonic conductivities of several different Nafion and Dow samples immersed in sulfuric acid solutions at various temperatures. Conductivities on the order of 0.06-0.085 S/cm were reported for the acid-immersed Nafion samples at 22 °C. The immersed Dow membrane samples exhibited somewhat higher conductivity (0.13-0.14 S/cm) at this temperature. These data were reported for membranes in contact with a minimum concentration of 0.3 m H2SO4 and are dependent on the sulfuric acid concentration. This study [77] presents a model of water and ion distribution based on properties of pores in the ionomer. The model, which uses Poisson s equation to describe electric potential variation in the pore, is successful in describing experimental acid partitioning results. Other earlier reports of protonic conductivity in ionomeric membranes have been given by Slade et al. [72] and by Eisman [60]. 

Springer and others were the first to use detailed, experimentally derived diffusion and electroosmotic drag coefficients of water in Nafion in a model for steady-state water profile and the resulting protonic conductivity in the membrane of an operating PEFC [87]. The distribution of water in a PEFC at steady, state (at constant current and reactant/water fluxes) was calculated in this model by considering water flow through five regions of unit cross-sectional area within the fuel cell two inlet... 

Choi, P. Jalani, N.H. Datta, R. Thermodynamics and proton transport in Nafion II. Proton diffusion mechanisms and conductivity. J. Electrochem. Soc. 2005, 152 (3), E123-E130. 

Table 6 compares the conductivity of various polymers and electrolytes containing sulfonic acid groups. The intrinsic conductivity of Nafion is high and is very similar to other polymers containing sulfonic acid groups. The activation energy of proton conduction of Nafion is low in comparison with other polymers. 

For high temperature membranes, three avenues are being pursued a) synthesis of new ionomers, b) covalent modification of Nafion , and c) pol mier-inorganic composites. Some of the approaches aim to increase the attractive forces holding water within the membrane, enabling operation at low RH. Others seek to embody Bronstead bases as an alternative to water, which allow for proton mobility in the absence of water. This mechanism is exemplified by proton conductivity in e.g. H3PO4 or CSHSO4, in which the protons hop and the protonated base need not move macroscopic distances. 

Shown in Figure 6 is a diagram of a nickel support and a Pt-loaded nanoparticulate oxide acting as a porous cathode. Organic-based fuel cell cathodes often have some Nafion added to improve proton conductivity but at the cost of electrical conductivity. However, we have the reverse problem - we have excellent proton conductivity in the inorganic cathode, but the oxides are electrical insulators. We need to boost the electrical conductivity of the cathode in order to fabricate a total inorganic MEA. This is a major objective of our future work. 

Among the proton-conducting membranes, Nafion or Nafion-like sulfonated perfluorinated polymers should also be mentioned. These materials are used for polymer electrolyte membrane (PEM) FCs, and in addition to being chemically very stable, they exhibit high proton conductivity at temperatures lower than 100°C. It is believed that permeability and thermal stability may be increased if tailor-made lamellar nanoparticles are added to a proton-conducting polymer. The sulfonated poly(ether ether ketone) (S-PEEK) type of polymers is also widely reported as an alternative to fluorinated polymers such as Nafion or Hyflon [51]. 

PFSA membranes possess high proton conductivity in the range of 0.1 S cm" at 80°C and good chemical as well as mechanical stability. The superior stability of Nafion is a consequence of the PTFE-based structure that is chemically... 

Figure 12.1. The dependence of proton mobility on water content, (a) Proton selfdiffusion coefficients (D

Proton conductivity in PEMs is usually a strong fimction of the degree of hydration. The maximum conductivity in Nafion ( 0.1 Scm ) is attained with water contents A, 15 (H2O SO3" ratio) imder typical operation conditions. Conductivity decreases monotonically towards lower water content. It usually exhibits a quasi-percolation transition for A, < 5 [21,22]. 

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